This invention relates to a process for calcium fluoride production from industrial waste waters. More specifically, it relates to the production of calcium fluoride from industrial waste waters by lime precipitation and carbonation.
Significant concentrations of inorganic fluorides are found in industrial waste water from glass manufacturers, electroplating operations, steel producers and certain chemical plants. These concentrations may be excessive by environmental standards and must be removed prior to discharge of the effluent water into the environment. Lime softening or treating of water for fluoride removal is a well-known and established process for fluoride reduction. However, low purity calcium fluoride produced by such a process cannot be sold and is itself a disposal problem. Such processes use a considerable excess of lime and require acidification, often with hydrochloric acid, to lower the pH to about 7 to 9 in order that effluent water disposal will not be a hazard. In addition, hydrochloric acid neutralization results in products which cannot be easily separated due to gel formation.
U.S. Pat. No. 3,957,902 discloses a process for the recovery of hydrogen fluoride prior to disposal of waste HF alkylation process gases containing hydrogen fluoride. In this process, a hydrocarbon liquid selected to be essentially free of hydrogen fluoride contacts the waste gases, absorbs hydrogen fluoride and returns it to the process. This patent also describes as common in the art the treatment of waste gases with calcium hydroxide to precipitate calcium fluoride in the form of a sludge, which is then disposed of as waste. It is additionally noted therein that disposal of the resultant precipitate sludge poses an inconvenience to the processor.
U.S. Pat. No. 3,964,993 discloses a process for removing hydrogen fluoride from the sludge resulting from the HF treatment of hydrocarbons which comprises concurrently treating the sludge with aqueous caustic and a liquid aromatic hydrocarbon.
U.S. Pat. Nos. 4,145,282 and 4,226,710 describe a process for purifying waste water containing fluoride which comprises precipitating fluoride from the waste water as fluorapatite. In this process, fluoride is removed as fluorapatite by having sufficient available calcium for each mole of fluoride present in the waste water and sufficient available phosphate for each mole of fluoride in the waste water to form a precipitate which is or converts to sufficient fluorapatite to remove essentially all fluoride from the water.
Japanese Kokai (laid open) No. 78999/78 describes a process for treating waste hydrogen fluoride or a waste gas containing hydrogen fluoride. According to this process, the hydrogen fluoride is first neutralized with caustic potash and then treated with slaked lime to precipitate calcium fluoride. The calcium fluoride slurry thus obtained is neutralized with an aqueous mineral acid to a pH of 7 to 9. A high molecular weight flocculant is then added to the slurry to form a floc, which is subsequently dehydrated to give a calcium fluoride mass. One disadvantage of this process is the fact that the waste water filtrate contains highh halide levels. The major disadvantage of this process is the formation of a gelatinous precipitate of calcium fluoride which tends to clog vacuum filters, thus making recovery of calcium fluoride difficult.